In order to provide a television with a screen size greater than approximately 40 inches a display device other than a direct view cathode ray tube (CRT) is typically used. As the screen size of a CRT increases, so too does the depth. It is generally accepted that for screen sizes greater than 40 inches direct view CRTs are no longer practical. Three alternatives exist for large screen (>40 inch screen size) displays: projection displays, plasma displays, and Liquid Crystal Displays (LCDs).
Current plasma and LCD displays are much more expensive than projection displays. Plasma and LCD displays are generally thin enough to mount on a wall, but can be heavy enough that mounting can be difficult. For example, current 42-inch plasma displays can weigh 80 pounds or more and 60-inch plasma displays can weigh 150 pounds or more. One advantage of plasma and LCD displays over current projection displays is that they are typically much thinner than current projection displays having the same screen size.
Projection displays, specifically rear projection displays, are typically more cost-effective then plasma displays. Projection displays may also consume too much space in a room to provide a practical solution for large screen needs. For example, typical 60-inch rear projection displays are 24 inches thick and can weigh 200 to 300 pounds.
FIG. 1 illustrates a prior art rear projection display device. In general, display device 100 includes optical engine 140, projection lens 130, back plate mirror 120 and screen 110. Optical engine 140 generates an image to be projected on screen 110. Projection lens 130 projects the image from optical engine 140 on to back plate mirror 120, which reflects the image to screen 110. The size of display device 100 is proportional to the size of the image to be displayed on screen 110. Thus, for large screen sizes (e.g., >60 inches), the overall size of display device 100 can be very large.
Fresnel lenses may be used to direct a projected image toward a viewer. Conventional rear projection display devices are thick because of surface reflections from the Fresnel surface. As the angle of incidence increases (on the flat side of the Fresnel) the amount of light that is reflected from the air-plastic interface also increases, reducing image uniformity. A person of ordinary skill in the art is familiar with calculating Fresnel surface reflections. The dimensions and arrangement of the surfaces in a Fresnel lens determines the angles at which the light will exit. Thus, any distortion to the shape of a Fresnel lens will distort the appearance of an image leaving the lens. Screen flex distortion is a type of distortion to the shape of a Fresnel lens. Screen flex distortion refers to distortion caused by pressure applied to the surface of a Fresnel lens that deforms the shape of the lens.
A Fresnel lens may also be distorted by the way it is mounted into a cabinet or otherwise implemented in a display device. Typically, a Fresnel lens is made of a thin sheet of flexible material. The thinness and the flexibility make the Fresnel lens difficult to handle and more difficult to maintain in a desired orientation, such as the vertical and flat orientation of the screen 110 of FIG. 1. The thin sheet of material comprising a Fresnel lens may be attached at a number of points to a mount in a rear projection display device to maintain the proper orientation. However, the tension in the Fresnel lens near the mounting connection points is typically greater than the tension in, for example, the center of the Fresnel lens. The uneven tension in Fresnel lens 120 may distort the shape of the lens and the image leaving the lens.
In some rear projection display devices, screen 110 may include both a Fresnel lens and a diffusion screen. However, both the Fresnel lens and the diffusion screen may be made of flexible materials. In these embodiments, the Fresnel lens and the diffusion screen both need structural support. Moreover, when the Fresnel lens is positioned adjacent to the diffusion screen, pressure applied to the surface of the diffusion screen may be transmitted to the associated Fresnel lens causing screen flex distortion.
To prevent screen flex distortion caused by pressure transmitted via a diffusion screen, some rear projection systems implementing Fresnel lenses, include an outer screen. The outer screen is separated from the diffusion screen by a distance that is sufficient to prevent contact between the screens when reasonable pressure is applied to the outer screen.
The pressure on, and deflection of, the outer screen does not cause screen flex distortion. However, the separation of the outer screen may cause parallax for users viewing the image on an angle. This is especially problematic when the outer screen is a touch screen, writing screen, or other screen configured for interactive input.